Covered hopper car types have been used extensively for chemical and petrochemical products. Also such covered hopper cars are used for handling grain. Products that are perishable or require protection from contamination and weather, such as food products, have also contributed to the growth of covered hopper cars. Covered hopper cars also still have some use for the cement service.
The covered hopper car makes it possible to ship materials in bulk that were formerly handled in bags or barrels. Generally, they have replaced the box car with grain doors in the bulk movement of that commodity. Such cars are often made with fully insulated structures and equipped with refrigeration for the bulk movement of perishables. Humidity control equipment is installed on some cars for the protection of chemicals and food products.
Many covered hoppers haul polyethylene plastic pellets or other petrochemical products which constitute the raw materials of finished plastic products. These cars have interior linings which are corrosion resistant and can be cleaned to prevent contamination.
The present invention is directed to the mechanical and structural features of a novel type of hopper car without regard to the particular types of material to be carried.
In the design of a high performance covered hopper car, it is desirable to reduce the forces at the wheel to rail interface. The static forces will not significantly change unless there is an increase in the number of axles carrying the load. Generally, if more trucks are employed, a better load distribution is achieved and lower static wheel forces will result. The problem then becomes controlling the dynamic loads. There are vertical and lateral dynamic loads that must be considered. The present invention is directed primarily to the vertical dynamic loads.
Excessive dynamic vertical wheel loads can come from several sources. The most notable source is the "rock-and-roll" problem which can be severe enough to cause wheel lift and de-railments. The major source of the rock-and-roll problem is the high center of gravity of a loaded covered hopper coupled with poor track conditions and truck centers. Having the truck centers less than 39 feet will also tend to inhibit rock-and-roll because this is generally the distance between rail joints.
Lowering the roof line on a hopper car will reduce the center of gravity of the car. However, if the car is made the same length with the lowered roof line, its load carrying capacity will decrease. If the roof line is lowered and the car made longer, then the excess length of the car will make it difficult and sometimes impossible to negotiate turns when there are obstructions along sides of the road bed.